FIG. 6 is a block diagram of a conventional type of narrow-band communication apparatus. The narrow-band communication apparatus based on the prior art employs a digital modulation technology and also employs a simplex system, and the receiving section is based on a triple super heterodyne configuration.
In FIG. 6, the conventional type of narrow-band communication apparatus largely comprises an RF and IF receiving section for a triple super heterodyne reception, a local frequency generating section for receiving, a local frequency generating section for transmission, a reference frequency generating section, a transmitting section, a signal processing section for digital modulation/demodulation, an audio output section, and a microphone audio input section.
The RF and IF receiving section for a triple super heterodyne receiver comprises a receiving frequency (RF) amplifier 601, a first mixer 602, a first intermediate frequency (IF) amplifier 603, a second mixer 604, a third mixer 605, a third intermediate frequency (IF) amplifier 606, a filter 607 and an A/D converter 608.
The local frequency generating section for receiving comprises a first local oscillator 613, a second local oscillator 614, a third local oscillator 615, and an amplifier 621 for AGC. Also the local frequency generating section is realized by a local oscillator 620 for transmission, and the reference frequency generating section is realized by a reference frequency oscillator 619.
The signal processing section for digital modulation/demodulation is realized by a transmitting/receiving digital modulator/demodulator 609 using, for instance, a DSP (Digital Signal Processor).
The audio output section comprises a D/A converter 610, an audio amplifier 611, and a loudspeaker 612. Also the microphone audio input section comprises an A/D converter 625, a filter 626, a microphone 627, and an amplifier 628 for a microphone.
The transmitting section comprises a transmission frequency (RF) power amplifier 622, a modulator 623, a D/A converter 624, a push-to-talk switch (PTT switch) 629, and a transmitter control circuit 630.
In a narrow-band communication apparatus employing digital modulation/demodulation having the configuration as described above, current in the digital signal processing section 609, A/D converter 608, D/A converter 610, and AID converter 625 is large, and power is dissipated even though transmission or reception is not always being executed, which causes a serious problem especially in a portable terminal unit using batteries or the like.
To overcome the problems as described above, for instance, in Japanese Patent Laid-Open Publication No. 345330/1992 is proposed a technique for lowering power consumption by executing so-called intermittent receiving in which a receiving system is intermittently operated, or by detecting a electric field strength and making the ON/OFF cycle longer when the detected electric field strength is weak. Another described technique disconnects power when electric field strength in a stand-by state is weak for a specified period of time.
However, the technique for lowering power consumption, as described above, is applied only to a receiving system, and in a narrow-band communication apparatus employing digital modulation/demodulation and having the configuration as described above, dissipation power in the transmission system is large as swell, and effective power saving has not been achieved. Also, detection of electric field strength has been used only for control over intermittent receiving or control of power disconnection, and efficiency of the circuit is low.
Also, in the conventional type of narrow-band communication apparatus as described above, when AFC (Automatic Frequency Control) is executed, as shown in FIG. 7, a pilot signal inserted at a center of an aural signal is used to measure the frequency.
The audio signal is shifted by 225 Hz respectively at a center of a band as shown in FIG. 8, and if a search is executed for AFC in a state where a strong audio signal or data exists near a pilot signal, sometimes the audio signal or data may be mistaken for a pilot signal because of a relation between a filter's bandwidth and the shift rate of an auding signal.
This problem can be solved by setting the shift rate of an audio signal to a large value, but the maximum effective band is limited to 4 KHz (in case of 220 MHz in USA), so that, if the shift rate is set to a large value, the audio signal can not be transmitted appropriately.
Also, the above problem can be solved by making the filter band for a pilot signal narrow, but then another problem occurs that timing for locking to the pilot signal becomes difficult. In brief, a finer search is required, and furthermore the response speed of the filter becomes slower, and consequently a longer time is required for searching.
In a tone-in-band receiver, generally demodulation of an audio signal is executed by referring to a pilot signal. Namely, a constant audio output can be obtained regardless of the electric field strength by measuring power P of a pilot signal and multiplying the demodulated aural element by 1/P. This is so-called AGC (automatic Gain Control) operation.
FIG. 4A shows characteristics of audio output and SINAD (signal plus noise plus distortion to noise plus distortion ratio) against electric field strength. As shown in the figure, audio output at a constant level is obtained because of AGC.
However, in a weak electric field, SINAD is low, so that, if the audio output is provided at a constant level, signals containing heavy noise are outputted at a constant level and it causes hearing discomfort. In addition, sometimes measurement of power of the pilot signal can not be executed correctly, and consequently noise level is raised as indicated by a dotted line in the figure. The phenomenon as described above also occurs when a receiving signal drops out due to Rayleigh fading or the like.
In the AGC operation, measurement of signal power is executed for a pilot signal having passed through a narrow-band filter for a pilot signal as described above, so that 100 ms or more is required only for, for instance, response by the pilot signal filter and a long time is required before the AGC operation is started.
Furthermore when a strong input is received, a pilot filter can not pass the pilot signal through before the AGC operation is locked, and for this reason the AGC operation is not started and the receiving section becomes saturated.
In brief, the conventional type of narrow-band communication apparatus has the following problems. First, an intermittent cycle is controlled or power is turned ON/OFF according to electric field strength detected during intermittent reception in a receiving system to reduce dissipation power in the apparatus, but this technique for reducing dissipation power has been applied only to a receiving system. In the conventional type of narrow-band communication apparatus employing an digital modulation/demodulation, any effective measure for power saving has not been applied to a transmission system although dissipation power in the transmission system is also large. Detection of electric field strength has been applied only to intermittent reception control or to power ON/OFF control, and consequently, circuit efficiency is rather low.
Second, if a search is executed, when executing AFC, in a state where a strong audio signal or data exists near a pilot signal, sometimes the audio signal or data may be mistaken for the pilot signal because of the relation between the filter bandwidth and audio signal shift rate. If the shift rate is set to a large value to overcome this problem, the audio signal can not be transmitted appropriately because there is a limit to the maximum effective bandwidth. If the bandwidth of a pilot signal is made narrower, timing for catching a pilot signal becomes difficult. This makes it necessary to execute a finer search, and consequently, the response speed of the filter becomes slower, and a longer time is required for searching.
Third, when AGC is executed according to the measured power of a pilot signal, an audio output at a constant level is obtained through AGC, but in a weak electric field region, SINAD is low. Hence, signals containing heavy noise are generated at a constant level, which causes serious hearing discomfort. Also pilot signal power can not be measured correctly, so that the noise level is raised, which makes hearing discomfort more serious.
Fourth, in the AGC operation, signal power measurement is executed for a pilot signal having passed through a pilot signal filter, so that a long time is required before the AGC operation is started. When, for instance, a strong input is received, the pilot signal can not pass through the pilot signal filter before the AGC operation is locked, and the AGC operation is not started, so that the receiving section becomes saturated.